The present invention is directed at a hydrotreating process for lube oils. More specifically the present invention is directed at an improved two-stage hydrotreating process for producing process oils from naphthenic feeds utilizing standard hydrotreating catalysts and equipment.
Naphthenic-rich feeds normally have lower wax contents, lower pour points, lower Viscosity Indices and higher ring contents than paraffinic-rich feeds. These properties make it desirable to utilize naphthenic-rich oils as process oil.
Naphthenic feeds, which often are utilized in the manufacture of process oils, frequently contain color bodies and undesirable impurities such as sulfur and basic nitrogen (heteroatom) compounds. The concentration of these compounds must be substantially reduced to meet product specifications. In addition, polynuclear aromatic compounds (PNA) also are present in naphthenic feeds. The concentration of these compounds also must be substantially reduced. The most common method for reducing the concentration of these compounds in lube oils is by contacting the feed with hydrogen in the presence of selected catalysts at elevated temperature and pressure.
Currently, naphthenic process oils are produced by a variety of process schemes including distillation only, distillation followed by mild acid treating and clay percolation or contacting, distillation followed by mild or severe extraction, mild or severe hydrotreating or combinations thereof. The milder processing conditions may produce process oils that are deficient in product composition and/or field performance. Typical measures of product composition are sulfur, basic nitrogen, polars, aromatics, neutralization number, ultraviolet levels of dimethyl sulfoxide extracts and the aniline point. Important product characteristics include compatibility with elastomers and solubility with a range of additives. It has been found that both the crude source and the processing severity affect these properties. Severe processing can drastically reduce product yields to uneconomic levels. The severity of the operating conditions also typically involves an economic balance of equipment availability and cost, yield and desired properties.
Several publications disclose two stage hydrodesulfurization with intermediate product removal. Japanese patent publication No. 71-003267 discloses the production of a highly viscous lubricating oil by passing the oil over a hydrotreating catalyst at 340.degree.-370.degree. C., removing hydrogen sulfide, ammonia and hydrogen followed by passing the product from the first stage through a second stage maintained at a temperature of 200.degree.-340.degree. C. This patent discloses the use of a two stage hydrotreating system operated over different temperature ranges with intermediate removal of hydrogen sulfide, ammonia and hydrogen. The process was utilized to produce a combination of gasoline, middle distillate and only a minor amount of lubricant basestock.
U.S. Pat. No. 3,884,797 discloses a two stage process for pretreatment of naphtha feedstocks prior to reforming to produce gasoline. The first stage comprises a hydrotreating zone operated at 500.degree.-850.degree. F. and at a pressure of 300-3,000 psig. The second stage comprises a hydrosorption zone operated at a temperature of 575.degree.-800.degree. F. and a pressure of 100-800 psig. The product from the hydrosorber is passed directly to a reforming zone operated at a temperature ranging between about 750.degree. F. and 1050.degree. F., preferably between about 850.degree. F. and 1000.degree. F. This process is not especially applicable to the production of lube base-stocks, since, at these conditions significant quantities of the lube feeds would be converted to coke and gas.
East German Pat. No. 59,354 discloses a two stage hydrotreating process in which the first stage hydrotreating is conducted at 350.degree.-450.degree. C. at a pressure of 150-300 atmospheres. After the gaseous products are separated, the second stage hydrotreating is conducted at 300.degree.-400.degree. C. and a pressure of up to about 300 atmospheres. The catalyst in both stages was an oxide or sulfide of Group VI or Group VIII. The use of such a process would not be desirable because of the relatively high pressures utilized. At these pressures, excessive hydrogenation would result in saturate levels and aniline points too high for process oils.
U.S. Pat. No. 3,349,027 also discloses the use of a multi-stage hydrodesulfurization process using typical catalysts with intermediate gas removal. Suitable operating ranges for both stages include the following: temperature 400.degree.-750.degree. F.; pressure 400-700 psig; and hydrogen 200-4,000 SCF/B. This patent does not address the removal of PNA's or maintaining the saturates below predetermined levels.
Other patents disclose two stage hydrotreating processes in which the second stage is operated at a lower pressure than the first stage. For example, UK Pat. No. 1,476,428 discloses a process for the manufacture of white oils, a class of oils having a very low aromatic content. The first stage is operated at a temperature of 300.degree.-425.degree. C., a hydrogen partial pressure of 10-250 bar (140-3600 psig), a space velocity of 0.1-5 kg per liter of catalyst per hour and a hydrogen/feed ratio of 100-5,000 Nl of hydrogen per kg of feed (500-25,000 SCF/B). The second stage treatment may be conducted at a temperature of 175.degree.-325.degree. C. with the ranges of the hydrogen partial pressure, space velocity and hydrogen/feed ratio being similar to those for the first stage. The catalyst for the first stage comprises a sulfided nickel and/or cobalt and molybdenum or nickel and tungsten. The second stage catalyst may be either the same catalyst used in the first stage or noble metal catalysts. The use of such a method for the production of a process oil would not be desirable, since the method would be relatively costly and would result in an almost completely saturated oil.
U.S. Pat. No. 3,928,168 discloses processes for the manufacture of hydrorefined oils under mild (below 800 psig hydrogen) and severe (above 800 psig) hydrotreating conditions to reduce sulfur and nitrogen contents. This patent discloses at column 9 that mild hydrotreating frequently does not significantly alter the polycyclic aromatic content of the oil.
East German Pat. No. 56,885 discloses a two stage hydrotreating process for the production of reformer feeds, diesel oils, household heating fuels and turbine fuels. Conventional hydrotreating catalysts such as cobalt molybdate/alumina, nickel molybdate/alumina or nickel sulfide/tungsten sulfide typically are used for the first and second stages. The first stage is conducted at temperatures of 300.degree.-450.degree. C., a liquid hourly space velocity (LHSV) of 1-10, the hydrogen feed ratio is 100-1,000:1 with a typical first stage pressure being 40 atmospheres. The second stage conditions may be as follows: temperature 200.degree.-370.degree. C., LHSV 0.5-15, and hydrogen/feed 100-1,000:1. A typical pressure also is 40 atmospheres.
U.S. Pat. No. 3,022,245 discloses a two stage hydrotreating process for the production of high quality wax to reduce color and odor. The temperature in the second stage is maintained lower than the temperature in the first stage. The temperature in the first stage typically is maintained between 500.degree. and 650.degree. F., with the temperature in the second stage maintained at least 100.degree. F. lower than the first stage. Pressure in both stages may range between 400 and 1,000 psig. The hydrogen treat rate is 200-750 SCF/B. The feed rates to the first and second stages are 3-5 v/v/hr, and 1-2 v/v/hr, respectively.
U.S. Pat. No. 3,208,931 discloses a two stage process for refining petroleum utilizing conventional hydrotreating catalysts. The patent discloses an example in which the first stage temperature was 750.degree. F. and the second stage temperature was 600.degree. F. The pressure was maintained at 1,000 psig in both stages. Space rates in the first and second stages were 0.3 v/v/hr and 0.49 v/v/hr, respectively, while the gas rates were 2,000 SCF/B and 8,500 SCF/B, respectively.
Other patents which disclose two stage hydrotreating processes include U.S. Pat. Nos. 2,771,401; 3,072,564; 3,089,841; 3,155,608; 3,717,501; 3,208,931; UK Pat. No. 1,546,504; and French Pat. No. 2,073,228.
While it is desirable to hydrogenate polynuclear aromatics, it is desirable to retain mono-aromatic ring compounds, since these mono-aromatic compounds promote improved solubility in the final oil product. To minimize the capital and operating costs of the system, it also is desirable to operate at relatively low pressures and at relatively high throughputs while simultaneously obtaining a high overall product yield. Therefore, the operating conditions which are selected frequently must produce a trade-off in one or more of these properties.
Accordingly, it is desirable to provide a process which is capable of producing a process oil having a relatively large mononuclear aromatics content while having sufficiently reduced polynuclear aromatics, sulfur and nitrogen contents.
It also is desirable to provide a process which is capable of producing a process oil at relatively high yields.
It also is desirable to provide, at moderate pressure, a process which is capable of being utilized in existing hydrotreating equipment.
It also is desirable to provide a process which can be utilized at relatively high throughput rates and at relatively low operating temperatures and pressures.
The present invention is directed at a method for producing a process oil having reduced sulfur, basic nitrogen, and polynuclear aromatics content from a naphthenic feed at relatively high through-put rates while only moderately decreasing the unsaturates content.
The present invention is directed at passing the feed sequentially through a first hydrotreating zone, an intermediate stripping zone and a second hydrotreating zone. The temperature in the second hydrotreating zone is maintained lower than the first hydrotreating zone temperature. The saturates and/or unsaturates content of the product exiting the second hydrotreating zone is monitored. The temperature in the second stage is adjusted and/or the catalyst is regenerated and/or replaced to keep the saturates content and/or the polynuclear aromatics content below predetermined limits.